1. Field of the Invention
The invention relates to an intra-aortic balloon pump having improved automated electrocardiogram (ECG) based intra-aortic balloon deflation timing. More particularly, the invention relates to an intra-aortic balloon pump capable of basing the decision of automatically activating and deactivating non-predictive deflation upon a quantitative assessment of the predictive performance of the intra-aortic balloon pump for the prevailing cardiac rhythm.
2. Description of the Prior Art
It is well-known in the art, as described in, for example, the specification of U.S. Pat. No. 4,362,150, to provide cardiac assistance by introducing a balloon into the thoracic aorta of a patient and causing the balloon to inflate and deflate in anti-phase with the contraction of the patient's heart. A balloon of this type is inflated at the beginning of diastole, in order to increase the blood flow to the coronary and carotid arteries. The balloon is then deflated just prior to the start of systole, in order to reduce the load on the left ventricle. It is essential that cardiac activity be sensed reliably to ensure that the balloon is inflated and deflated accurately with respect to the cardiac cycle.
Methods of sensing cardiac activity include analysis of aortic pressure and/or analysis of the electrocardiogram. It is known in the art, as described in U.S. Pat. No. 5,169,379, to combine means for effecting such analysis with the aforementioned intra-aortic balloon (IAB) apparatus.
The focus of the present invention is the automatic control of deflation timing of the intra-aortic balloon. Using the ECG as a time-base, a maximum reduction in end diastolic pressure is achieved when IAB deflation begins in advance of the start of the next cardiac cycle, i.e. R-wave. This deflation modality will hereinafter be referred to as "predictive" deflation since the start of the next cardiac cycle must be predicted, based on prior beat intervals. The goal of predictive deflation is to predict the start of the next cardiac cycle and to completely deflate the balloon in advance of the next predicted beat. Algorithms for predicting the start of the next cardiac cycle for a regular cardiac rhythm are generally known in the art of balloon pumping.
One difficulty with using a standard predictive deflation algorithm for control of the intra-aortic balloon is the potential onset of cardiac rhythm variations. In the presence of random and chronically irregular rhythms, such as atrial fibrillation, accurate prediction of the next ECG beat is not possible. Prediction can be made with only limited statistical probability. Accordingly, such random dysrhythmic patterns are generally managed by having the intra-aortic balloon pump deflate the intra-aortic balloon on the leading edge of the R-wave. This method of intra-aortic balloon deflation will hereinafter be referred to as R-wave deflation. R-wave deflation is a non-predictive deflation method which produces a later deflation of the intra-aortic balloon than that produced by predictive deflation. The advantage of setting the intra-aortic balloon pump to R-wave deflation mode in the presence of an irregular rhythm, however, is that deflation of the intra-aortic balloon begins precisely upon the identification of the next R-wave, regardless of the variance of the rhythm. This enables the intra-aortic balloon pump to consistently augment the entire diastolic interval and unload the next impending left ventricular contraction.
It is not necessary that all rhythm variations be managed by switching to R-wave deflation. For example, algorithms are known in the art for identifying transient disturbances (dysrhythmia) such as premature ventricular complexes (PVCs), including isolated PVCs and Couplets, and also for recognizing sudden changes in heart rates. These rhythm variations can be rapidly identified and are typically followed by a predictable beat pattern. Accurate prediction of such beats, after a brief initial learning phase period, is often possible.
As indicated above, intra-aortic balloon pumps presently on the market are automated. The intra-aortic balloon is controlled by a predictive algorithm in situations involving regular rhythms and at least one pump will automatically adopt R-wave deflation upon degradation of the rhythm beyond a threshold level. Bard'S TRANSACT IABP, for example, incorporates an algorithm for determining when to switch to and from R-wave deflation, which is based upon beat-to-beat, i.e. R-R interval, variability.
More specifically, if the Bard intra-aortic balloon pump detects a large beat-to-beat variation in cardiac interval for 8 out of the last 16 beats intervals then the Bard pump abandons the use of predictive deflation and adopts R-wave deflation control. A major drawback to this method is that the decision to abandon the predictive mode is independent of the success that the intra-aortic balloon pump is having in following the timing variations associated with the rhythm disturbance. For example, if a patient's rhythm was such that a premature ventricular contraction occurred on every other beat, and the Bard intra-aortic balloon pump is designed to successfully track this rhythm, the Bard pump would still abandon the predictive mode because the above described requirements have been met, i.e at least 8 out the past 16 R--R intervals were sufficiently variable to trigger the switch from the predictive deflation mode to the R-wave deflation mode.